As Per me This Must Be a Simple Task
According to what I understand about Your
Need
Try This and Please Get Me know your Feedback.
Let us Suppose Form1 is your Support Form Containing
The dataGridview having the Related Data
And Form2 be Your main Form Containing the TextBox
Inside Form1 means in your Support Form
under dataGridView_CellContentClick
Write The Following Code like
MainformName.TextBoxName1.Text=dataGridView.SelectedCells.Item(0).Value.ToString()
MainformName.TextBoxName2.Text=dataGridView.SelectedCells.Item(1).Value.ToString()
Implement this As Per Your need